1. Technical Field
This invention relates to a method of molding glass materials into optical elements and optical elements molded by the method.
2. Background of the Art
When heated up to a temperature about twice as high as the glass-transition temperature (Tg), glass materials for forming optical elements melt and become fluid to drip by gravity. For example, an optical glass material of a glass-transition temperature (Tg) of 500° C. can melt and drip through a nozzle when heated at 1000° C. or higher (which is twice as high as the glass-transition temperature). In this melting temperature range, glass can change its form freely. By putting a glass material in a mold for an optical element, we can transfer a mold shape approximately exactly to the glass material. For example, Patent Document 1 discloses such a droplet molding method of forming glass droplets for optical elements.
Patent Document 1: Japanese Examined Patent Publication H04-16414
However, in the droplet molding method of Patent Document 1, the molds may be exhausted quickly since the high-precision and smooth optical transfer surface of the mold is frequently in touch with extremely hot molten glass, easily coupled with oxygen in the air, and deteriorated. The optical surface of a mold made of, for example, cemented carbide material will be rough by some thousand molding shots and have to be replaced by a new one. Consequently, we must stop the molding machine for die change and set up the new mold. This takes a lot of time, cuts the operating ratio of the machine greatly and as the result reduces the productivity of the molding machine. Further, the cemented carbide mold materials are extremely hard and it takes a lot of time and trouble to machine the high-precision optical transfer surface of the mold. Therefore, it is difficult to say that the droplet molding method (which will easily deteriorate the high-cost molds) is fit for mass production of low-cost glass optical elements.
A re-heating molding method has been proposed and in practical use which comprises the steps of putting a glass piece of room temperature in the cavity of a mold, heating the mold and the glass piece together, press-molding the molten glass in the mold, cooling the mold and the glass piece together after the press-molding is completed, and taking out the shaped glass optical elements after the glass becomes fully hardened. This method has problems of occupying the mold which has a greater heat capacity than the glass piece during both heating and cooling the glass piece, keeping the mold at a constant heating rate to heat the glass piece of bad heat conductance uniformly, and having a longer molding tact because of low thermal coefficient and utilization efficiency. Therefore this method is not adequate to mass production of glass optical elements.
To solve these problems, the conventional reheating molding methods have contrived to shape a plurality of optical elements at a time (multiple-molding) or to prepare a lot of molds and pass them through a heating furnace, a pressing furnace, and a cooling furnace in the order continuously. Since these methods, however, heat the glass pieces and the molds together from the outside, the molds become basically hotter than the glass pieces in the molds, the molten glass sticks to the optical transfer surface of the mold. This reduces the operating rate and reliability of the molding machine. Also in the above method that passes molds through furnaces, it is very difficult to assure both a high reliability of molds and high-precision molding of glass optical elements since placing a pair of molding parts having opposite optical transfer surfaces without eccentricity is inconsistent with accomplishing a high tolerance in fitting of parts to slide the molds smoothly for pressing.
It appears to be a reality that there has been no better molding method which can reduce the mold occupying time and molding time and produce high-precision glass optical elements at high reliability and yields than these conventional molding methods.